Effect of melatonin on baroreflex control of heart rate in rabbits

The effect of intravenous injection of melatonin [0.1 mg/kg, 0.5 mg/Kg and 1 mg/Kg] on arterial blood pressure and heart rate in adult male rabbits was investigated in this study. Baroreflex function was assessed using phenylephrine [the pressor test] to induce moderate changes in arterial blood pressure and to alter the stimulation of baroreceptor sites. In addition, the effect of melatonin on sympathetic and parasympathetic efferent nerve activity was also investigated. Melatonin resulted in a lowering effect on arterial blood -pressure with a decrease in heart rate. The significant decrease of arterial blood pressure was observed at a dose as low as 0.5 mg/Kg. The lowering effect of melatonin on heart rate was detected at 1 mg/Kg. Baroreflex slope was not depressed significantly with a significant increase in pulse interval, until I mg/Kg melatonin. Sympathetic and vagal nerve activities were attenuated significantly by melatonin. This study indicated that baroreflex control of heart rate was depressed by intravenous administration of melatonin. There was a dissociation between the effect of melatonin on baroreflex sensitivity versus the effect of low blood pressure. Melatonin was found to alter the baroreflex heart rate response through its direct depressant effect on baroreceptors and on both sympathetic and parasympathetic efferent nerve pathways

Mechanism of decreased pressor responsiveness of estradiol in pregnant rabbits

The mechanism of decreased pressor responsiveness to oestradiol during pregnancy was examined in rabbits. Oesdtradiol injection resulted in a rise in mean arterial pressure [MAP] greater in nonpregnant than pregnant animals. Rabbits 25 days pregnant, showed marked blunting of the pressor response to oestradiol, whereas after 5 days of pregnancy there was a normal response and at 15 days an intermediate pressor response. Following treatment with indomethacin, sensitivity to the hypertensive action of oestradiol was perceptibly increased only in pregnant rabbits. Prostaglandin F2alpha [PGF2alpha] injection casued a significant decrease in MAP and vascular sensitivity [mean diastolic pressure] greater in nonpregnant than in pregnant rabbits. Plasma level of sodium was significantly increased after oestradiol and showed a nonsignificant change after PGF2alpha. Plasma level of potassium showed a nonsignificant change after the injection of oestradiol and PGF2alpha. Histological examination of renal or adrenal tissues of rabbits injected with oestradiol showed a picture similar to those of pregnant animals. Juxtaglomerular apparatus and zona glomerulosa showed signs of increased activity. There were no significant histologidal changes in the renal or adrenal tissues after PGF2alpha injection in oestradiol-treated rabbits when compared with those treated with oestradiol only. The only detect able change was the greatest increase in vascular sensitivity. These observations showt hat the altered responsiveness to oestradiol in pregnant rabbits appears to be - related to hyporesponsiveness angiotensin II and to an increased synthesis of vasodilator prostaglandins. There may be two of the mechanisms by which the maternal circulation adapts to pregnancy